Milbemycin, or B-41 is a substance which is isolated from the fermentation broth of a milbemycin producing strain of Streptomyces. The microorganism, the fermentation conditions and the isolation procedures are more fully described in U.S. Pat. No. 3,950,360 and U.S. Pat. No. 3,984,564. The milbemycin compounds described in said patents do not have any substitution at the 13-position.